There are many applications for high speed low distortion feedback amplifiers with current output stages. One such application is a transconductance amplifier used in an Ethernet interface chip for tapping into a coaxial cable in an Ethernet system. The interface chip receives a digital signal and converts it to a current in the coax cable. Because of the Ethernet electrical specifications relating to rise time and fall time, frequency components in excess of 10 MHz must be amplified and converted with low distortion to an output current having a range of 0 to 90 milliamps.
Many transconductance amplifiers, while able to pass high frequencies, have stability compromises due to the changes in transconductance (gm) or gain in the output or feedback transistors as a function of the current levels through these transistors. Since the typical Ethernet interface requires variations in output currents of approximately 16 to 1, which results in approximately a four to one change in transconductance in a MOS transistor or a 16 to 1 change in a bipolar transistor, robust compensation techniques are required to provide stability in the interface circuit.
Miller compensation is traditionally used to improve the stability of voltage amplifiers. Miller compensation provides for an amplifier bandwidth that is independent of output transistor transconductance Unfortunately, Miller-compensated amplifiers normally have low output impedance at frequencies approaching the unity gain frequency of the amplifier. The Ethernet application demands a current output amplifier with high output impedance.
Therefore, it can be appreciated that a transconductance amplifier with compensation circuitry to provide stable operation of the amplifier at high frequencies and over a wide range of operating currents is highly desirable.